Sounds like you are having quite an adventure. What are the tight margins you mentioned? Fitting margins? If you are getting that close, and it really matters, then finding a good fitter should be your next move. However, having experienced riders with you giving immediate feedback as you ride is also valuable. They can watch your leg extension, rocking hips, back profile, arm profiles, etc. And for sure no one but you knows when the fit is really really comfortable. I took my own advice, shorted the cockpit significantly and no more feeling stretched out. Also then lowered the bars. Same with the seat, pedal clips, hood positions. Ride until you notice something is off, no matter how far that is, and then adjust.

And once I get it all set up, I'll probably go in for a fitting just to see what changes the fitter suggests. Might be educational. And might just be backwards.

Doing all your own maintenance is as you said, priceless. Even if you take the bike into the LBS, you will know when the job is muffed or done right.